1. Field of the Invention
The present invention relates to an electro-conductive composition used as an underlayer for a solder, for example, in soldering an electronic component on a printed circuit board or the like fabricated by printing an electro-conductive composition composed of electro-conductive particles such as metallic particles or the like and an electronic equipment using the same.
2. Description of the Related Art
In soldering an electronic component on a printed circuit board fabricated by printing an electro-conductive composition, for example, it is important that not only does a solder have a good wettability to an conductive layer of a printed circuit board with which an electronic component is connected, but a molten solder is prevented from penetrating into the conductive layer beyond a depth in soldering. Such incompatible actions are hard to be simultaneously exerted in a controllable manner in the same printed electro-conductive layer and it has in this regard been considered that direct soldering is difficult on a metal-resin composition used in an electro-conductive layer or the like on a printed circuit board. In order to make it possible to conduct soldering on an electro-conductive layer, the inventors has conceived a method in which a first composition mainly composed of metallic particles having a good wettability and a second composition mainly composed of metallic particles having a poor wettability are used as an underlayer of the solder in a superposing manner, like as the top layer composed of metallic powder which has good wettability with molten solder over the bottom layer composed of metallic powder which has poor wettability with molten solder. Since the compositions are required in use to be formed in a sequentially layered manner, there is a necessity for two compositions to be coated on an electro-conductive path on a circuit board in two applications. For example, in the case where a fine pattern is fabricated by screen process printing or the like, there have been problems that it is hard to conduct two-layer printing using the compositions without misalignment and the number of fabricating steps increase in such a process. For this reason, the inventors have developed a composition with which underlayer preparation for soldering by formation of an electro-conductive composition layer can be completed with a single application.
However, if the first composition and the second composition are simply mixed with each other, there arises no problem in wettability, but such a mixture is poor in a blocking action against penetration of a molten solder. For this reason, a solder penetrates through the mixture as a underlayer to reach a metal-resin composition printed as an circuit forming electro-conductive layer, whereby the metal-resin composition comes to solid like a metal. In such a condition, the metal resin composition loses a resin property at the interface between the composition and a board and as a result, an adhesive force of the metal-resin composition to the board becomes insufficient, so that, though it depends on circumstances, there has been a fear that the metal-resin composition, that is the electro-conductive layer, is separated from the board.